1. Field of the Invention
The present invention relates to rolling parts and power transmission parts formed of carbon steel produced through induction hardening.
2. Description of the Background Art
Conventionally, rolling bearings, a typical rolling part, have often been formed for example of SUJ2 or other similar high-carbon chromium bearing steel, SCM420 or other similar, case-hardened steel carburized, or the like. They are sufficiently reliable steels for bearings. However, they are expensive as they contain Cr, Mo and other expensive elements, and using such elements also results in consumption of rare resources and thus desirably should be avoided. In particular, when SCM420 and other similar case-hardened steels are thermally processed they are required to be heated for a long period of time, or carburized, and this also consumes a large amount of thermal energy.
In contrast, in recent years automobile leg bearings and constant velocity joints (CVJ), ball screws, and any other similar rolling parts also sliding as they roll are formed of S53C or other similar medium carbon steel, with the rolling portion alone induction-hardened. Medium carbon steel has an alloy element content smaller than the afore-mentioned bearing steel and case-hardened steel and it is inexpensive and has satisfactory workability. It is disadvantageous, however, as it is inferior in important characteristics, i.e., it has a shorter rolling life.
To overcome the above disadvantage, conventionally members have been increased in size to alleviate load or surface pressure and they have thus been used without problems. In the future, however, energy conservation and miniaturization will result in high surface pressure acting on such members and a longer rolling fatigue life is thus demanded. Furthermore, CVJs, ball screws and the like can have a rolling portion also sliding as they roll, and they are thus also required to have long life accordingly. Furthermore, miniaturization requires that members be reduced in thickness and that a raw material itself corresponding to a non-hardened portion also have enhanced fatigue strength.
Furthermore as a result of the miniaturization of the part a large amount of heat is emitted and confined and the entirety of the part is thus exposed to higher temperature than conventional.
Thus, energy conservation, miniaturization and the like result in a rolling portion being used under severer conditions and a material providing a long rolling life is thus demanded. As has been described previously, CVJs, ball screws and the like can have a portion sliding as it rolls. As such they are required to have a long life not only as a simply rolling part but a rolling part which also slides. Thus the xe2x80x9crolling lifexe2x80x9d as aforementioned refers not only to that of a rolling part that simply rolls but that of a rolling part also sliding as it rolls
In addition to the above demand, miniaturization and associated reduction of members in thickness entail acceptance of relatively large load. As such, a raw material itself of a non-hardened portion is also required to have larger fatigue strength. In order to in crease the life of the exact raw material without increasing the cost thereof it is effective to increase the contents, as represented in percentage, of C, Si, Mn or any other similar, inexpensive alloy element in conventionally used medium carbon steel. In other words, increasing these inexpensive alloy elements in amount enhances the strength of the raw material and hence the fatigue strength the exact raw material.
If the raw material is excessively hardened, however, it would be inferior in workability. The present invention is directed to a rolling part having a complex shape, for example thread cutting. As such, turnability, forgeability, pierceability and other similar working characteristics are also important. Thus, high-carbon steel and high-alloy steel, such as bearing steel, are unsuitable. Case-hardened steel that is carburized is also unsuitable for the above rolling parts as it needs to have a treaded portion protected against carbonization and its complex shape facilitates over-carburization and if boundary oxidization occurs under mill scale the steel can be impaired in strength. To enhance workability, the raw material can have its hardness adjusted for example by a quenching and tempering process after it is cast and molded. To reduce the cost of the raw material, however, desirably the quenching and tempering process is excluded and the raw material that is not quenched or tempered is processed.
There can be obtained a rolling part and power transmission part excluding any expensive elements as its constituents and formed of inexpensive elements C, Si and Mn optimized to allow the same to have characteristics equivalent to those of rolling parts using bearing steel, and a power transmission part using the rolling part. The above characteristics are as follows: An application miniaturized and thus incapable of accepting temperature elevation, requires a rolling life allowing for its high temperature use.
(a) Rolling portion corresponding to an induction-hardened portion:
(a1) rolling life thereof as it simply rolls
(a2) rolling life thereof as it slides while rolling
(b) Non-hardened portion
(b1) limit of typical fatigue characteristics, or rotating bending fatigue
(b2) workability
The present invention in one aspect provides a rolling part formed of steel containing 0.5 to 0.8% by weight of C, 0.5 to 1.2% by weight of Si and 0.3 to 1.3% by weight of Mn and having a surface hardness of no less than HRC 59.
The steel contains 0.5 to 0.8% by weight of C to ensure that induction-hardening provides a surface hardness of no less than a predetermined value. It contains C with a lower limit of 0.5% by weight to ensure a long rolling life with a large load imposed while Si, Mn and the like are contained, as predetermined. Carbon forms carbide and to obtain steady hardness larger carbon contents, as represented in percentage, are preferable. Too high carbon contents, as represented in percentage, however, impair cold-workability, and a soaking process for prevention of component segregation, spheroidization of carbide, and other similar, particular heat treatments are required, which is costly. To ensure good cold-workability and dispense with soaking, C has an upper limit set to be 0.8% by weight.
The steel contains 0.5 to 1.2% by weight of Si because Si is an element increasing a rolling life and it also prevents the steel from softening when it is exposed to high temperature, and it acts to delay microstructural change, cracking, and the like attributable to large load applied repetitively. Medium carbon steel containing 0.5 to 1.2% by weight of C, as provided in the present invention, and containing less than 0.5% by weight of Si, cannot exhibit its effect and provides a rolling life increasing as no less than 0.5% by weight of Si increases. More than 1.2% by weight of Si, however, significantly impairs cold-workability and hot-workability and increases production cost. Si thus has an upper limit set to be 1.2% by weight.
0.3% by weight of Mn contained in the steel improves the steel in hardenability and it dissolves into solution in the steel to enhance the steel in toughness and also increases retained austenite beneficial in increasing a rolling life. Mn, however, as well as Si, reinforces a raw material and if its content as represented in percentage is too high it impairs workability and machinability. Mn thus has an upper limit set to be 1.3% by weight.
Desirably Al is low in level to ensure a long rolling life, although it is not necessarily required to be particularly low if it has approximately a normal level, and so is P.
As the above alloy components integrally act, the steel can be produced in the same process line as conventional carbon steel and provide a material providing a long rolling life. Of the above alloy elements, C, Si and Mn contribute to providing inexpensive medium carbon steel having a rolling life close to that of bearing steel and workability close to that of carbon steel. The above-described steel forming the present rolling part that is induction-hardened and tempered, can obtain hardness of no less than HRC 59. The steel for example induction-hardened ensures hardness more reliably than S53C or other similar, typical carbon steel induction-hardened, as the former contains the alloy elements C, Mn and Si increased in amount and thus has high hardenability.
In the above first aspect desirably the rolling part is formed for example of the steel containing C, Si and Mn, as represented in percentage, satisfying the following expressions (1) and (2):
L=11271 (C wt %)+5796 (Si wt %)+2665 (Mn wt %)xe2x88x926955xe2x80x83xe2x80x83(1) 
Lxe2x89xa75000xe2x80x83xe2x80x83(2) 
wherein L represents an estimated index of a rolling life obtained through multiple regression analysis. In the present invention, C, Si and Mn are limited by the condition L25000 as provided above. Thus, with hardenability and the like enhanced, hardness ensured, and the like, a further increased rolling life can be provided.
In the above first aspect desirably the rolling part is formed of the steel containing for example no more than 0.02% by weight of Al and no more than 0.02% by weight of P.
Al forms an oxide-based, non-metallic inclusion and it thus has a negative effect on a rolling life. In particular, C, Si, Mn-based steel, as used for the present rolling part, is more disadvantageously susceptible to non-metallic inclusion. No more than 0.02% by weight of Al is thus desirable.
P segregates at grain boundary and reduces toughness. As such, with austenite phase being low in state, no more than 0.02% by weight of P is desirably used to provide a long rolling life and enhanced fatigue strength.
The present rolling part is produced in a method including the steps of: processing in a predetermined shape a steel at least containing 0.5 to 0.8% by weight of C, 0.5 to 1.2% by weight of Si and 0.3 to 1.3% by weight of Mn; and induction-hardening a member processed in the step of processing. Introducing C, Si, Mn in an appropriate range more in amount than S53C or other similar, typical medium carbon steel, can maintain excellent workability and in addition ensure high hardenability. As such, the present steel can be readily processed on the same process line as conventional carbon steel and it also readily ensures a high level of hardness through induction-hardening. Furthermore, increasing Si in amount mainly can provide a high level of strength for high temperature. Thus a high yield of rolling parts can be produced efficiently.
Desirably in the present method for example the step of induction-hardening is followed by the step of tempering the member to provide a surface hardness of no less than HRC59. The steel used to form the present rolling part has a composition with Si increased in amount and thus highly resistant to softening attributable to tempering. Thus it readily ensures hardness if it is quenched and tempered at (a) relatively high temperature or (b) the same temperature for a long period of time.
The present invention in a second aspect provides a rolling part formed of steel shaped and thus processed, at least containing as alloy elements 0.5 to 0.7% by weight of C, 0.6 to 1.2% by weight of Si and 0.6 to 1.0% by weight of Mn, as represented in percentage, to satisfy the following equations (1) and (2):
Lxe2x89xa75000xe2x80x83xe2x80x83(1) 
wherein L represents an estimated lifetime in regression calculated from a measured value of life as the rolling part simply rolls and
L=11271 (C wt %)+5796 (Si wt %)+2665 (Mn wt %)xe2x88x926955; and 23xe2x89xa6Hxe2x89xa625xe2x80x83xe2x80x83(2) 
wherein H is an estimated value of the following equation:
H=48.0 (C wt %)+5.7 (Si wt %)+11.5 (Mn wt %)xe2x88x9216.2 
in hardness of a raw material, as calculated from a measured value of the hardness of the raw material.
In the above configuration the chemical composition has a range set for the following reason: no less than 0.5% by weight of C is required for allowing induction-hardening to ensure hardness of no less than a predetermined value and for ensuring a satisfactory rolling life with a large load imposed while Si and Mn are contained invariable in amount. Thus C has a lower limit set at 0.5% by weight. C forms carbide and to constantly ensure hardness larger amounts of C are preferred, although more than 0.7% by weight of C would result in raw material having too high levels of hardness and impair workability. Furthermore, a soaking process for prevention of component segregation, a carbide spheroidization process, and other similar, particular thermal treatments would also be required, which is costly. No more than 0.7% by weight of C is thus set.
0.6% by weight of Si contained in the steel reinforces raw material to provide a long rolling life and also prevents it from significantly softening when it is exposed to high temperature. Furthermore the Si thus contained acts to delay microstructural change, cracking, and the like attributable to large loads applied repetitively. It also does not contribute to increasing the raw material in hardness so much as Mn described hereinafter. More than 1.2% by weight of Si impairs cold-workability and hot-workability. No more than 1.2% by weight Si is thus set.
0.6% by weight of Mn contained in the steel improves hardenability and it dissolves into solution in the steel to enhance the steel in toughness and also increases retained austenite beneficial in increasing a rolling fatigue life. Mn, however, as well as Si, reinforces a raw material and it also dissolves into carbide and thus increases hardness thereof and hence that of the raw material. Thus more than 1.0% by weight of Mn impairs workability and machinability. Mn thus has a range set to be 0.6 to 1.0% by weight.
The above estimated value of life set to be 5,000 is required in order to provide an induction-hardened portion with a rolling fatigue life L10 of no less than 5,000 multiplied by 104.
Furthermore if the raw material has an estimated value H in hardness of no less than 23 a non-hardened portion that is not quenched or tempered can have a rolling and bending fatigue strength of no less than 400 MPa. For H exceeding 25, however, it would be hardened excessively and thus impaired in workability. H of no more than 25 is thus set. These levels of hardness may be that of a material of a portion hardly varying with a low-temperature tempering process and free from an effect of induction-hardening that has been tempered in a low temperature range.
In the above, second aspect the rolling part can include a hardened portion provided by the steel shaped and thus processed that is induction-hardened.
Employing induction-hardening to provide a hardened portion can provide a rolling part with a rolling portion having a long rolling fatigue life and a long rolling and sliding fatigue life.
In the above, second aspect desirably the material of the portion free from the effect of induction-hardening has a 107-time fatigue limit of no less than 400 MPa, as measured in a rotating bending fatigue.
Thus the steel can obtain a level of fatigue strength more than 30% higher on average than conventional medium carbon steel and thus endure severe conditions in use expected in the future, such as large load, large torque, and miniaturization. Note that the portion free from the effect of induction-hardening for example includes (a) raw material (before it is neither shaped nor induction-hardened), (b) a portion induction-hardened and tempered and yet free from the effect of induction-hardening, and (c) a portion induction-hardened and not yet tempered, and free from the effect of induction-hardening.
In the above, second aspect desirably the rolling part at a rolling portion corresponding to an induction-hardened portion can have a rolling life L10 of no less than 5,000 multiplied by 104 times, as measured in a rolling fatigue test, and a life longer than S53C, as measured in a rolling and sliding fatigue test.
As such if rolling and associative sliding stress is applied the rolling part can have sufficient durability.
A power transmission part including any one of the above rolling parts can have a satisfactory level of workability, a long rolling life and a long rolling and sliding life, and it also can have a non-hardened portion having superior fatigue characteristics. Thus it can have high durability and also be inexpensive.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.